The invention relates to a system for detecting an unauthorized attempt to tap the optical fiber of an optical communications system. Tapping an optical fiber line usually requires extracting some power from it. Conventional tap detectors measure the received power and produce an alarm if the received power is less than expected. The present invention improves upon prior optical fiber tap detectors by modifying the original data signal such that the transmitted signal includes synchronizing periodic waveform information as well as a fixed total power. The synchronizing information is detected in the receiver and used to produce a local signal which is combined with the transmitted signal. When the combined signal is out-of-balance as a result of extraction of power due to tapping of the optical fiber, an alarm is activated.